Prior to the present invention, various one and two-package moisture curable room temperature vulcanizable (RTV) compositions were available based on the use of a silanol-terminated polydiorganosiloxane having the formula, ##STR1## where R is a C.sub.(1-13) monovalent substituted or unsubstituted hydrocarbon radical, which is preferably methyl, or a mixture of a major amount of methyl and a minor amount of phenyl, cyanoethyl, trifluoropropyl, vinyl, and mixtures thereof and n is an integer having a value of from about 50 to about 2500, with a cross-linking silane having hydrolyzable radicals attached to silicon.
For example, Ceyzeriat, U.S. Pat. No. 3,133,891 and Bruner, U.S. Pat. No. 3,035,016, are based on the use of methyltriacetoxysilane with a silanol-terminated polydimethylsiloxane under substantially anhydrous conditions. Although the one-package compositions of Bruner or Ceyzeriat, upon exposure to atmospheric moisture, provide satisfactory one-package room temperature vulcanizable organopolysiloxane compositions exhibiting satisfactory tack-free time, for example, 30 minutes or less after an extended shelf period, the acetic acid by-product is corrosive and has a disagreeable odor.
Other variations of one-package acyloxy acid generating RTV's are shown by Kulpa, U.S. Pat. No. 3,296,161, Goossens, U.S. Pat. No. 3,296,195 and Beers, U.S. Pat. No. 3,438,930, assigned to the same assignee as the present invention. Additional one-package acyloxy acid generating RTV compositions are shown by Schulz et al, U.S. Pat. No. 3,647,917 and Nitzsche et al U.S. Pat. No. 3,886,118.
An improved, low odor, substantially non-corrosive one-package RTV composition is shown by Beers, U.S. Pat. No. 4,257,932, assigned to the same assignee as the present invention. Beers achieves a reduction in odor and corrosive properties by utilizing as a crosslinking silane a less volatile material such as methyltris-(2-ethylhexanoxy)silane.
A non-corrosive two package moisture curable organopolysiloxane composition free of carboxylic acid generating groups is shown by Nitzsche et al, U.S. Pat. No. 3,127,363 which is based on the use of a polyalkoxysilane, or polysilicate cross linking agent, in place of methyltriacetoxysilane. The ingredients of the two package noncorrosive composition of Nitzsche et al, are mixed under atmospheric conditions and the resulting composition must be used soon after the ingredients are mixed because the resulting blend has a short shelf life. Although the mixture of Nitzsche et al, which is typically polyalkoxysilane, silanol-terminated polydiorganosiloxane and tin soap catalyst, provides upon mixing, a fast curing non-corrosive room temperature vulcanizable composition, the Nitzsche et al mixture does not have the extended shelf life advantage of the one package system which is required for commercial use and therefore is excluded from a variety of applications.
Nitzsche et al, U.S. Pat. No. 3,065,194, teaches that a mixture of an endblocked dimethylsiloxane polymer, such as hydroxy and alkoxy endblocked, inert filler, ethylorthosilicate and dibutyltindilaurate can be vulcanized upon contact with water, after a 14 day shelf period at room temperature. However, the various ingredients of the mixture have to be vigorously dried by heating for 1 hour at 200.degree. C., and the RTV, after a relatively short shelf period, has to be drenched with water.
Improved results toward combining the advantages of a non-corrosive acid-free polyalkoxysilane cross-linking agent with a silanol-terminated polydiorganosiloxane as a one-package system are shown by Weyenberg, U.S. Pat. No. 3,334,067, Cooper et al, U.S. Pat. No. 3,542,901 and by Smith et al U.S. Pat. Nos. 3,689,454, and 3,779,986, the last two being assigned to the same assignee as the present invention, utilizing a titanium chelate catalyst in place of a tin catalyst. However, after room temperature vulcanizable one-package systems based on a titanium chelate catalyst were allowed to age for a period of 5 hours or more, it was found that the tack-free time of the aged RTV was considerably longer than the tack-free time of the same mixture after it was initially mixed and immediately exposed to atmospheric moisture.
As shown by Brown et al U.S. Pat. No. 3,122,522 a platinum catalyst is used to prepare an alkoxy terminated silalkylenepolysiloxane polymer. However, this method of synthesizing the base polymer requires an expensive hydrosilylation procedure. Additional efforts to achieve a desirable non-corrosive, substantially odor-free stable one-package RTV based on the use of polyalkoxy organopolysiloxane in a more economic manner are shown by Brown et al, U.S. Pat. No. 3,161,614 or U.S. Pat. No. RE 29760. Brown et al employed a polyalkoxy end blocked polysiloxane which was based on the use of a mineral acid generating polyalkoxyhalosilane, and a curing catalyst. However, these compositions were found to be unusable because they failed to cure in contact with a tin catalyst, even in the presence of moisture.
As utilized hereinafter, the term "stable" as applied to the one package polyalkoxy-terminated organopolysiloxane RTV's of the present invention means a moisture curable mixture capable of remaining substantially unchanged while excluded from atmospheric moisture and which cures to a tack-free elastomer after an extended shelf period. In addition, a stable RTV also means that the tack-free time exhibited by freshly mixed RTV ingredients under atmospheric conditions will be substantially the same as that exhibited by the same mixture of ingredients exposed to atmospheric moisture after having been held in a moisture resistant and moisture-free container for an extended shelf period at ambient conditions, or an equivalent period based on accelerated aging at an elevated temperature.
The present invention is based on the discovery that stable, substantially acid-free, one-package, moisture curable polyalkoxy-terminated organopolysiloxane RTV compositions can be made by using a silanol terminated polydiorganosiloxane consisting essentially of chemically combined diorganosiloxy units of the formula ##STR2## such as a silanol-terminated polydiorganosiloxane of formula (1), where R is as previously defined, with an effective amount of certain silane scavengers for chemically combined hydroxy radicals. In the silanol-terminated polydiorganosiloxane consisting essentially of chemically combined formula (2) units, the presence of silicon bonded C.sub.(1-8) alkoxy radicals such as methoxy radical is not precluded. The hydroxy radicals which can be removed by the silane scavenger can be found in materials normally present in the RTV composition of the present invention, for example, trace amounts of water, methanol, silanol radicals on the silica filler (if used), the silanol polymer of formula (1), or a silanol-terminated polymer having formula (2) units. The silane scavenger useful for eliminating chemically combined hydroxy radicals in accordance with the practice of the invention preferably has the formula, ##STR3## where R.sup.1 is a C.sub.(1-8) aliphatic organic radical selected from alkyl radicals, alkylether radicals, alkylester radicals, alkylketone radicals, alkylcyano radicals or a C.sub.(7-13) aralkyl radical, R.sup.2 is a C.sub.(1-13) monovalent organic radical selected from R radicals as previously defined, and defined more particularly below, X is a hydrolyzable leaving group selected from amido, amino, carbamato, enoxy, imidato, isocyanato, oximato, thioisocyanato, and ureido radicals. The preferred members are amino, amido, enoxy, and the more preferred is amido, for example, N-C.sub.(1-8) alkyl-C.sub.(1-8) acylamido, a is an integer equal to 1 or 2 and preferably 1, b is a whole number equal to 0 or 1 and the sum of a+b is equal to 1 or 2. In formula (3), where a is 2, X can be the same or different. The leaving group X reacts preferentially before --OR.sup.1 with available --OH in the RTV composition and provides an RTV composition substantially free of halogen acid, or carboxylic acid. The silane scavenger of formula (3) is both the silane scavenger for hydroxy functional groups and a polyalkoxysilane cross-linking agent for terminating the silicon atom at each organopolysiloxane chain-end with at least two alkoxy radicals.
Among the ingredients of the RTV compositions which are formed as a result of the use of the hydroxy scavenger of formula (3), is silanol-free polydiorganosiloxane, chain-terminated with two or three --OR.sup.1 radicals. The silanol-free polydiorganosiloxane optionally can be combined with an effective amount of a cross-linking silane, as defined hereinafter, under substantially anhydrous conditions. The cross-linking polyalkoxysilane which can be utilized in combination with the scavenging silane of formula (3) has the formula, ##STR4## where R.sup.1, R.sup.2 and b are as previously defined. The preferred condensation catalysts which can be used in the practice of the invention include metal compounds selected from tin compounds, zirconium compounds, and titanium compounds or mixtures thereof. Additional condensation catalysts which can be used are defined more particularly hereinafter.
It is not completely understood why the polyalkoxy-terminated organopolysiloxane compositions of the present invention are stable in the presence of certain condensation catalysts over an extended period of time in the substantial absence of moisture.
A mechanistic study of the RTV of the present invention supports the theory that the use of scavenging silane of formula (3) or (5) below or in combinations thereof with crosslinking silane of formula (4), in accordance with the practice of the invention, minimize the likelihood that detrimental amounts of R.sup.1 OH will be generated during the shelf period. R.sup.1 OH generation is to be avoided because R.sup.1 OH endstops the silanol polymer of formula (1) or polymer with formula (2) units to produce polymers having terminal ##STR5## units. These polymers, wherein the silicon atom at each polymer chain end is terminated with only one alkoxy radical, have slow cure times. In addition, R.sup.1 OH can breakdown the organopolysiloxane polymer in the presence of the condensation catalyst.
The aforedescribed one package RTV compositions of Brown et al are unstable. In Brown et al, which uses chloroalkoxysilanes, the chloro radical is the leaving group. It has been found that chloro radical leaving groups can form highly corrosive halogen acids, or halogen acid amine salts, when used with an acid acceptor, such as pyridine.
The use of the silane scavenger for hydroxy of formulas (3) or (5), in which the leaving group X is not a halogen radical, substantially eliminates undesirable water in the filler and silicone polymer, as well as residual moisture in the RTV composition during the shelf period. In determining what level of silane scavenger of formula (3) or (5) to use in the practice of the invention, the total hydroxy functionality of the RTV composition can be estimated. The total hydroxy functionality of the polymer can be determined by infrared analysis. In order to insure that an effective or stabilizing amount of scavenger is used to maintain the stability of the composition over an extended shelf period of six months or more at ambient temperature while in a sealed container, there can be used an additional amount of scavenger over that amount required to endstop the polymer. This excess of scavenger can be up to about 3% by weight, based on the weight of the polymer. The aforementioned 3% of scavenger by weight exceeds that amount required to substantially eliminate available hydroxy functionality in the polymer as a result of reaction between OH functionality and X radicals. In compositions which also contain filler and other additives, the additional amount of scavenger of formulas (3) or (5) which is required is estimated by running a 48 hour stability check at 100.degree. C. to determine whether the tack-free time remains substantially unchanged as compared to the tack-free time of the composition before aging measured under substantially the same conditions.
Where polyalkoxy-terminated polymer of formula (6) below is made without using silane scavenger of formula (3), silane scavenger can be used in the practice of the invention having less than two--OR.sup.1 radicals attached to silicon, as shown by the formula, ##STR6## where R.sup.1, R.sup.2, and X are as previously defined, c is a whole number equal to 0 to 3 inclusive, d is an integer equal to 1 to 4 inclusive, and the sum of (c+d) is equal to 3 or 4. In such situations, the scavenging silanes of formula (5) can be used in an amount sufficient to stabilize the RTV composition as previously defined for the scavenging silane of formula (3). In addition, there can be used with scavengers of formulas (3) or (5) at least 0.01 part and up to 10 parts of the cross-linking silane of formula (4).
The polyalkoxy-terminated organopolysiloxane of the present invention has the formula, ##STR7## where R, R.sup.1, R.sup.2, X, n and b are as previously defined and e is equal to 0 to 1 and the sum of b+e is equal to 0 to 1. The polyalkoxy-terminated organopolysiloxane of formula (6), can be made by various procedures. One procedure is taught by Cooper et al U.S. Pat. No. 3,542,901 involving the use of a polyalkoxysilane with a silanolterminated polydiorganosiloxane in the presence of an amine catalyst. A method not taught by Cooper et al is the use of the silane scavenger of formula (3) as an end-capper with silanol-terminated polydiorganosiloxane used in the practice of the invention.
In formulas (1-6), R is preferably selected from C.sub.(1-13) monovalent hydrocarbon radicals, halogenated hydrocarbon radicals and cyano alkyl radicals, R.sup.1 is preferably a C.sub.(1-8) alkyl radical or a C.sub.(7-13) aralkyl radical, R.sup.2 is preferably methyl, phenyl, or vinyl.
The preferred X radicals in formulas 3, 5 and 6 are amido, amino and enoxy, and the most preferred is amido.
The expression "substantially acid-free" with respect to defining the elastomer made from the RTV composition of the present invention upon exposure to atmospheric moisture means yielding by-products having a pKa of 5.5 or greater with 6 or greater preferred and 10 or greater being particularly preferred.
It has been further found that improved cure rates can be achieved if minor amounts of amines, substituted guanidines, or mixtures thereof, are utilized as curing accelerators in the polyalkoxy compositions of the present invention. These curing accelerators also serve to catalyze the ability of the enoxy leaving group to act as a scavenger. There can be used from 0.1 to 5 parts, and preferably from about 0.3 to 1 part of curing accelerator, per 100 parts of the silanol-terminated polymer of formula (1), or which consists of chemically combined units of formula (2), or 100 parts of the polyalkoxy-terminated polymer of formula (6) to substantially reduce the tack-free time (TFT) of the RTV composition of the present invention. This enhanced cure rate is maintained after it has been aged for an extended shelf period, for example, 6 months or more at ambient temperatures, or a comparable period under accelerated aging conditions. Its cure properties after the extended shelf period will be substantially similar to its initial cure properties, for example, tack-free time (TFT), shown by the RTV composition upon being freshly mixed and immediately exposed to atmospheric moisture.
It appears that the curing accelerators described herein, in addition to decreasing the tack-free times of the RTV compositions of this invention, also provide a surprising stabilizing effect for particular RTV compositions catalyzed with certain condensation catalysts which exhibit a marked lengthening of tack-free time after accelerated aging. For this class of condensation catalysts, addition of amines, substituted guanidines and mixtures thereof described herein provide stable RTV compositions which exhibit a fast cure rate initially, i.e. less than about 30 minutes which remains substantially unchanged after accelerated aging.
The RTV compositions of the present invention can cure to a depth of 1/8" thickness within 24 hours. Durometer Hardness (Shore A) can then be determined and used to evaluate the cure of the compositions as shown in the examples.